U.S. Pat. No. 4,361,142 discloses a knee orthosis having a pair of knee joint assemblies 13 and 14 positioned on opposite sides of a patient's knee, each assembly having a femoral sidebar 15 and a tibial sidebar 16 extending alongside the upper and lower leg, respectively. The upper femoral sidebars are held along the upper leg by suspension means in the form of interfacial member 17, straps 20, 21, and pad 26 and, similarly, the lower tibial sidebars are immobilized along opposite sides of the lower leg by suspension elements in the form of interfacial members 18 and 19 and straps 22-24.
At the upper end of each tibial sidebar is a body portion 40 having a pair of planar side walls spaced apart to define a narrow upwardly-opening socket 38 that slidably receives the head portion 34 of a femoral member 30 of which the upper femoral sidebar 15 is an integral part (FIG. 2). Within the socket is a narrow arcuate guide surface 39 that engages the bearing surface 35 of the femoral member for guiding movement of the members between flexion and extension along constantly changing instantaneous axes of rotation. When the joint is in flexion as shown in FIG. 8, the head 34 would be capable of limited sliding movement posteriorly and anteriorly along the guide surface 39 in the absence of some additional constraining force. The interfitting relationship therefore assures stability at all degrees of flexion but there is nevertheless sufficient laxity in the mechanical joint to permit normal action of the patient's knee. Healthy ligaments are allowed to control natural knee movement. At the same time, at least one flexible but non-stretchable strap 33, and ordinarily a plurality of such straps (three are shown in FIG. 2), secured at selected points to the respective members of the orthotic joint, perform essential functions that would normally be performed by the injured or chronically deficient ligament, thereby protecting that ligament without interfering with the functions of the healthy ligaments. The positions and connections of the straps depend on the ligament or ligaments requiring protection or the stability required if such ligament or ligaments are absent. In any case, for any grouping of straps there is a sequential tensioning and untensioning of such straps during flexion and extension, with the result that the injured or deficient ligament is protected at all degrees of flexion.
The arrangement and positioning of straps 33 depend on the particular ligament or ligaments requiring protection. FIGS. 1-8 illustrate the attachment and positioning of straps for a patient with injury to or chronic deficiency of the posterior cruciate ligament. A primary function of that ligament in a normal knee joint is to pull the head of the femur in a posterior or rearward direction over the tibial articular surface during flexion and to hold it in a rearward position when the knee is fully flexed. The articulation of the femoral member 30 and tibial member 31 of the orthotic joint mimics the articulation of the natural joint, and the set of straps 33 is mounted to exert forces during flexion, as well as at extension and 90.degree. flexion, that mimic forces that would normally be exerted by a natural posterior cruciate ligament. For a patient whose condition requires protection of other major ligaments, the straps and their points of attachment would be different. For example, FIG. 9 depicts an orthotic joint for use with a patient having an injured or chronically deficient anterior cruciate ligament, and FIG. 10 illustrates a joint with strap attachment points arranged for protection of a collateral ligament.
While the orthosis briefly described above and disclosed in detail in U.S. Pat. No. 4,361,142 is considered superior to previous orthoses because, among other things, there is sufficient laxity in the mechanical joint to permit healthy ligaments to control natural knee movement, proper operation of the orthotic joint in protecting the injured or deficient ligaments requires precise adjustment of the length and location of attachment of the straps 33. Slight stretching of the straps over a period of extended use reduces the protective function of the orthosis. While problems of stretching might be reduced by forming the straps or cables of metal (as indicated in the patent), the constant flexing and tensioning of those straps or cables may nevertheless produce dimensional changes that require periodic inspection and possible adjustment or replacement. Wear on those straps or cables, and their points of connection is inevitable, especially in view of the exposed condition of those elements. Also, because the straps are exposed and bulge outwardly when they are untensioned, such straps are vulnerable to contact with objects or clothing that might interfere with their proper operation.